The present invention relates to the field of communication systems, and more particularly to an apparatus and method for providing gain equalization to optical signals carrying a plurality of wavelengths.
Various conditions in optical communication systems make it desirable to be able to selectively attenuate one or more particular wavelengths in an optical signal relative to other wavelengths in that signal. For example, it may be advantageous to provide a flat gain response across multiple wavelength channels of an optical signal. This typically requires providing separate attenuation circuitry for each wavelength to be attenuated. Although conventional systems exist for providing attenuation in optical signals, no system has emerged that provides cost effective attenuation in multiple wavelength systems. This problem becomes increasingly acute as optical systems strive to implement more and more wavelength channels.
One particular problem that can arise in an optical communication system supporting many wavelengths involves controlling the gain tilt in the transmitted signal. Existing optical communication systems have typically been limited to using the conventional (xe2x80x9cCxe2x80x9d) band of wavelengths to communicate optical signals. With the increasing demand for bandwidth, the capacity of communication systems is being expanded by the addition of new communication bands. For example, future communications systems will likely use the long wavelength (xe2x80x9cLxe2x80x9d) band and possibly even the short wavelength (xe2x80x9cSxe2x80x9d) band.
As additional wavelength bands are utilized and the net power in the fiber is increased, a problem can arise from an inter-channel Raman effect. In particular, longer wavelength channels can rob power from the shorter wavelength channels, creating a gain tilt after propagation through the fiber. The gain tilt can become increasingly pronounced as links of amplified fiber segments are cascaded.
The present invention recognizes a need for a method and apparatus operable to economically provide gain equalization in a multiple wavelength optical signal.
In one aspect of the invention, a gain equalizer comprises a wavelength division demultiplexer operable to separate one or more communication bands into a plurality of wavelengths and an array of phase shifter stages. Each phase shifter stage comprises a micro-electro-optic system (MEMS) device comprising a moveable mirror layer operable to receive a first copy of an input signal from a beam splitter and to reflect the first copy of the input signal for combination with a second copy of the input signal at an output to form an output signal. The moveable mirror layer is displaceable in a substantially piston-like motion to introduce a phase shift between the first and second signal copies at the output, the amplitude of the output signal varying depending on the displacement of the moveable mirror layer. The gain equalizer further comprises a wavelength division multiplexer operable to receive a plurality of phase shifted wavelengths from the second beam splitter and to multiplex at least some of the phase shifted wavelengths into an optical output signal.
In another aspect of the invention, a method of facilitating gain equalization of a plurality of wavelengths comprises receiving an optical input signal comprising a plurality of wavelengths, separating the optical signal into a plurality of wavelength signals, and communicating at least some of the wavelength signals to an array of attenuators. The method further includes, at each attenuator, communicating a first copy of the input wavelength signal toward a first reflective surface and a second copy of the input wavelength signal toward a second reflective surface, at least one of the reflective surfaces comprising a moveable mirror layer of a first micro-electro-optic system (MEMS) device. In addition, the method comprises reflecting the first and second wavelength signal copies toward an output, combining components of the reflected first and second wavelength signal copies to form at least one output wavelength signal, and displacing the moveable mirror layer in a substantially piston-like motion to result in an interference between the first and second wavelength signal copies at the output and a corresponding change in the amplitude of the output wavelength signal relative to the amplitude of the input wavelength signal.
In still another aspect of the invention, a multiple band optical communication system comprises a plurality of optical amplifiers coupled in parallel, each operable to receive and amplify one of a plurality of communication bands, each communication band comprising a plurality of wavelengths. The system further comprises a gain equalizer coupled to at least one of the plurality of optical amplifiers, the gain equalizer comprising a plurality of equalizing stages each operable to receive one of the plurality of amplified wavelengths and to selectively introduce attenuation or gain into each wavelength.
Depending on the specific features implemented, particular embodiments of the present invention may exhibit some, none, or all of the following technical advantages. One aspect of the present invention provides an effective and cost efficient mechanism for facilitating gain equalization in a multiple wavelength optical communication system. In a particular embodiment, implementing MEMS based attenuation stages facilitates fabrication of arrays of these devices at an incremental additional cost to fabricating a single attenuation stage. This aspect of the invention provides significant advantages in facilitating rapid, effective, and economical gain equalization, particularly in an environment supporting numerous wavelengths.
Another aspect of the invention provides an effective and cost efficient mechanism for dynamically adjusting the gain tilt in a multiple band communication system, while maintaining an acceptable optical signal to noise ratio.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.